[TowerTalk] GPS for azimuth

[jimlux]

On 1/23/17 4:28 AM, Steve Maki wrote:
> On 1/22/2017 22:25 PM, Jim Brown wrote:
>
>> For GPS to determine azimuth the observer must be moving. That works
>> fine in a vehicle, but not when you're walking.  Ask me how I know. :)
>
> Or you must have two GPS antennas in one unit, separated by some
> distance, usually 18" or more. We use such units to align cell antennas.
> They are supposedly accurate to one degree or better.
>
> I can't imagine how a stationary single antenna GPS can measure azimuth.
>
I'm not aware of any commercial systems that do this, but it is possible 
by designing the antenna to have a deliberately non uniform phase 
behavior as the spacecraft moves. That puts a "pseudo doppler" on top of 
the true doppler.  If you've got a position fix already, and you have 
the current ephemeris, you know what the doppler should be and where the 
spacecraft is in the sky.  By comparing the phase/doppler you measure 
with the phase/doppler expected, you can figure out the orientation of 
the antenna.

Much easier to just have 2 or 3 antennas separated by a couple of feet. 
That makes it easy to do attitude determination.  It was done quite a 
few years ago (more than 10-15) for determining artillery position and 
orientation and for input to aircraft autopilots for landing assistance.